- The anatomy of the heart
- What is atrial fibrillation?
- Are there different types of atrial fibrillation?
- Is atrial fibrillation common?
- What puts you at risk of developing atrial fibrillation?
The anatomy of the heart
The heart is a pump distributing blood to the entire body. It has 4 chambers. The upper chambers are the atria – a left and right atrium, the lower chambers are the ventricles – a left and right ventricle. The atria collect blood returning to the heart via the veins and with a gentle contraction fill the ventricles of the heart. The ventricles pump blood out of the heart via the arteries, this output is in rest about 4 to 5 liters per minute. With each beat the right ventricle sends blood to the lungs and the much bigger left ventricle sends blood to every other organ in the body.
The heart’s main electrical control, the ‘pacemaker’ is a group of cells called the “sinus node”. It sends out the normal regular electrical signal that is transmitted across the atria and then into the ventricles via the heart’s main relay station the “atrio-ventricular node”. This system of electrical transmission makes the heart muscles contract sequentially so that the atria first squeeze blood into the ventricle and then the ventricles pump blood to the body and the lungs.
The pulmonary veins bring oxygen rich blood from the lungs to the left atrium. Typically there are 4 of these, 2 from the right lung and 2 from the left lung. The area of their junction with the left atrium has abnormal electrical properties that has been shown to cause atrial fibrillation. This is why this part of the pulmonary veins are almost always the target for curative treatments such as ablation.
What is atrial fibrillation?
Atrial fibrillation is a condition that makes the heart beat irregularly unlike the normal heartbeat that is regular. The top chambers of the heart “the atria” quiver in a very rapid and chaotic pattern so blood flow slows down or stagnates. The main pumping chambers “the ventricles” also beat irregularly so they do not push blood around the body as efficiently.
Atrial fibrillation represents a problem with the heart’s electrics. A good analogy for this is the movement of water in a pond. If you drop a stone into one corner of a pond a neat ripple moves across the pond. In the same way, during a normal rhythm, electricity spreads smoothly from the pacemaker across the atria to the ventricles. If you drop stones randomly all over a pond you get choppy, messy waves moving in all directions. This is like the electrical waves in atrial fibrillation, which give the heart its chaotic and irregular pumping.
Why do the heart’s electrics malfunction like this? We do not fully understand why but many different diseases (Table 1) also damage the heart’s electrical properties and may cause atrial fibrillation. These diseases include not only diseases limited to the heart but also other disease like diabetes. Atrial fibrillation can also occur without a known or detectable heart condition. The heart cells located where the pulmonary veins join the atria are particularly vulnerable to behaving electrically abnormally. The pulmonary veins then are often the source of the chaotic electrical waves producing atrial fibrillation.
Table 1.
|
Acute reversible conditions |
Chronic conditions |
|---|---|
| Acute alcohol binge | High blood pressure |
|
Pulmonary embolism |
Heart attack |
| Pneumonia (infection of one or both lungs) |
Heart failure |
|
Pericardial effusion |
Heart valve disease |
| Pericarditis | Congenital heart disease |
| Cardiac Surgery |
Chronic alcohol abuse |
| Thyrotoxicosis | Obstructive sleep apnoea (cessation of airflow during sleep preventing air from entering the lungs caused by obstruction of the upper airways) |
| Obesity (excessive body fat with negative effects on health) |
Are there different types of atrial fibrillation?
If atrial fibrillation stops within a short time, then it is called paroxysmal but if your atrial fibrillation is present continuously or continues for prolonged periods then it is persistent. These terms are important because they are important factors in deciding on your suitability for the treatments available. When normal heart rhythm cannot be restored your atrial fibrillation is termed permanent (Table 2).
Table 2.
|
Paroxysmal |
Episodes of atrial fibrillation that last less than 7 days |
|
Persistent |
An episode of atrial fibrillation lasting longer than 7 days |
|
Longstanding persistent |
An episode of atrial fibrillation lasting longer than 1 year |
|
Permanent |
When no attempt is made or it is not possible to restore normal (sinus) rhythm. |
HOW COMMON IS AFIB?
Atrial fibrillation is the commonest abnormal heart rhythm and it is found in about 1% of the population. It is much more common as we get older. It is rare in young people but found in 5% of those over the age of 65 and 10% of those over 80.
WHAT PUTS YOU AT RISK OF DEVELOPING ATRIAL FIBRILLATION?
There are a number of medical conditions that increase the risk of developing atrial fibrillation during your life (table 1). Nearly every condition that increases the strain on the heart has been linked to atrial fibrillation.
Obstructive sleep apnoea is a condition where during sleep the muscles in the throat relax causing blockage of the airway. You tend to snore loudly and partners describe you gasping for air or even stopping breathing. This repeated starvation of oxygen is very bad for the heart and atrial fibrillation can result. Obesity has also recently been recognised to increase the risk of atrial fibrillation.
It is very important to recognise these underlying conditions favouring atrial fibrillation so that they can be properly treated. This may be as important as treating the atrial fibrillation itself. However, nearly 1/3 of people with atrial fibrillation have a normal heart and no clear underlying cause. This is described as “lone atrial fibrillation”.